1. Field of the Invention
The present invention relates to a method of processing molten aluminum and molten aluminum alloys (hereinafter referred to inclusively as “molten aluminum alloy(s)”), effectively using sludge from an aluminum alloy structure processing process using zinc phosphate, and a flux to be used by the method.
2. Description of the Related Art
Recycling of aluminum alloys have become prevalent in recent years in view of resource conservation and cost reduction. Since different alloys have different additive element contents, respectively, aluminum alloys are recycled in secondary aluminum alloys and foundry aluminum alloys having compositions which are easy to adjust.
Recovered scraps are melted in a molten metal by a melting process, the molten metal is refined by a refining process, slag is removed from the refined molten metal by a slagging-off process, and the refined molten metal is subjected to casting to provide secondary alloys or cast alloys. The melting process through the slagging-off process will be described. Materials, i.e., scraps, put in a melting furnace are heated and melted to produce a molten metal. Organic substances adhering to the materials, and oxides produced by the melting process form various inclusions in the molten metal. The inclusions and metals are mixed to form slag by the agency of a refiner, and the slag floats. The slag is a principal cause of defects in products and hence must be removed. However, since the slag contains metals in a high metal content, metal loss occurs if the slag containing metals is removed. Therefore, the metals contained in the slag must be returned to the molten metal as much as possible and only the inclusions must be efficiently removed by the slagging-off process.
A representative slagging-off process sprays a halogen flux over the surface of the molten metal such that the halogen flux and the slag are mixed. The halogen flux reacts with fine aluminum particles contained in the slag. This reaction generates reaction heat, the reaction heat heats the slag to enhance the fluidity of the metals contained in the slag so that the metals are returned to the molten metal. A generally known degassing process blows a flux together with an inert gas, such as nitrogen gas or argon gas, into the molten metal.
The degassing process and the slagging-off process are very important molten metal processing processes to clean the molten metal and to improve the quality of products. Slag disposal agents for removing slag from a molten aluminum alloy are proposed in, for example, JP-A Nos. 207376/1995 and 80851/1999.
The automobile industry uses a large amount of aluminum alloys for forming various members in view of weight reduction. Most automotive bodies, in particular, are formed from steel panels and aluminum alloy panels. Generally, those panels are subjected to a zinc phosphate treatment for surface preparation before coating. Since the steel panels and the aluminum alloy panels are subjected simultaneously to the zinc phosphate treatment, the aluminum ion concentration of a processing liquid employed in the zinc phosphate treatment increases gradually, and the processing ability of the processing liquid to process the steel panels deteriorates. To prevent such deterioration of the processing ability, a generally known method adds fluorine ions in the processing liquid to precipitate aluminum ions. Although this method enhances the ability of the zinc phosphate treatment, a large amount of sludge containing precipitated aluminum fluoride as a principal component is produced. This sludge has a high fluorine content and is difficult to use. Therefore, the sludge is disposed of as waste by landfilling.
Efforts have been made for the improvement of aluminum alloys to suppress the increase of sludge. For example, a method of suppressing the increase of sludge proposed in JP-A No. 256873/2000 forms a film over the surface of aluminum alloy member to control the elution of aluminum ions. A method that uses a fluoride is used prevalently owing to its cost effect and efficiency.
A special trader collects the sludge periodically and disposes of the sludge as industrial waste by landfilling or the like. Increase in the amount of aluminum alloys used not only increases waste disposal cost, but also entails a serious environmental problem of difficulty in finding places for waste disposal.
The inventors of the present invention made earnest studies to solve the foregoing problems. The inventors analyzed the composition of sludge produced by the zinc phosphate treatment of structures of steel and aluminum alloys and found that the sludge contains a large amount of components of cryolite (Na, Al, F). The respective contents of those components of cryolite are dependent on the fluoride concentration of a zinc phosphate treatment bath and the amount of processed aluminum alloy.
When an aluminum alloy is melted, a molten metal processing flux is used to eliminate hydrogen that forms pores when the molten aluminum alloy solidifies and to clean the molten aluminum alloy by eliminating nonmetallic inclusions that cause internal defects.
The inventors noticed that general fluxes for molten metal processing contains Na. The inventors made studies of using cryolite as a flux and found the use of sludge as a flux for molten aluminum alloy processing.